Petro, 51, has been a key player in Colombia's recent history and ran a "zero corruption" campaign in the nation's capital, whose last elected mayor is jailed in a bid-rigging scandal.

Five years ago, his denunciations as a senator of close ties between national and regional politicians and right-wing death squads spurred the so-called "parapolitics" scandal that has landed dozens of lawmakers in prison.

Short, slim and bespectacled, Petro is deliberate in speech and favors tweed jackets. Like many prominent Colombians unafraid to speak their minds, he has periodically been a target of death threats and has long been assigned a phalanx of bodyguards.

Petro, who finished fourth in last year's presidential election, won the mayor's race with 32 percent of the vote against 25 percent for his nearest challenger, Enrique Penalosa, with nearly all ballots counted in the city 8 million people.

Penalosa defeated Petro in 1997 for the same job, which has in the past been a springboard to Colombia's presidency.

Urban planners widely admire Penalosa for making Bogota more pedestrian- and cyclist-friendly during his term, and for launching a bus rapid transit system that has been a model in Latin America and beyond.

But analysts say Penalosa, 57, was hurt by his endorsement by conservative former President Alvaro Uribe in a city more friendly to the left.

"Bogota continues to be a fortress of electoral freedom," said analyst Alfredo Molano. "Gustavo Petro is a step forward in defeating machine politics."

Bogota is Colombia's biggest city, its urban area the Latin America's sixth most populous. Its gritty southern districts teem with tens of thousands of refugees from the country's long-running conflict.

The voting in the capital was part of nationwide regional and municipal elections, with 32 governorships and more than 1,100 mayoral and municipal council post being contested. Authorities and electoral watchdog groups reported relatively few voting irregularities.

Twenty soldiers had been killed in two separate attacks less than two weeks before the vote blamed on FARC rebels. Election-day FARC attacks are common but there were no immediate reports of rebel violence Sunday.

Regional and municipal elections tend to be a better barometer than presidential votes in Colombia of the relative health of the country's democracy. This year, illegal armed groups including the FARC and right-wing bands, both fortified by drug trafficking profits, intimidated candidates throughout rural Colombia.

Violence has been on the uptick since Juan Manuel Santos was elected president in mid-2010, and at least 42 candidates in local races were killed in the weeks leading up to Sunday's vote.

Petro, who begins his four-year term as mayor Jan. 1, has been harshly critical of the FARC, saying it is tainted by its involvement in drug trafficking and ransom kidnappng.

It is nothing, he says, like the M-19 movement that he joined as a student in the early 1980s, when he graduated from Bogota's prestigious Externado University with an economics degree.

Former comrades and relatives say Petro was never involved in violence, working instead to recruit and organize for M-19.

"He was a small, fragile, skinny person with myopia," his sister, Adriana, told The Associated Press in a 2007 interview.

M-19 was named for April 19, the date of the 1970 presidential election that many Colombians believe was stolen in favor of the Conservative Party candidate, Misael Pastrana.

Friends say the outrage expressed by Petro's mother over that outcome led him into leftist politics.

While the FARC, or Revolutionary Armed Forces of Colombia, is a mostly peasant guerrilla force formed in 1964, M-19 was a more classic Latin American rebel group formed largely by urban, middle-class intellectuals.

It became renowed for publicity-seeking actions including the theft of the sword of Latin American independence leader Simon Bolivar and the 1980 two-month takeover of the embassy of the Dominican Republic.

Petro was not personally involved in M-19's greatest fiasco: the 1985 takeover of the Palace of Justice in which more than 100 people, including 11 Supreme Court justices, were killed.

M-19's detractors contend the late drug lord Pablo Escobar financed the takeover. Petro vehemently denies this, and blames an unprovoked storming by the military for the deaths.

Captured 20 days before the raid, Petro still has scars from a week of torture in which he says he was shocked and beaten, denied food and almost drowned. He was jailed for a year and a half for rebellion.

After M-19 signed a peace pact with the government in 1990 and helped rewrite Colombia's constitution the following year, Petro was elected to Congress.

He spent most of the last two decades there and, after being elected senator in 2006 began revealing details of close collaboration between lawmakers and far-right militias known as "paramilitaries" that wound end up sending more than 60 politicians to prison from crimes ranging from criminal conspiracy to murder.

Last year, Petro helped uncover a bid-rigging scandal in Bogota that has landed its latest elected mayor, Samuel Moreno, in jail facing corruption charges.

The state found that some $1.2 billion in government funds had been diverted in the awarding of contracts including for the avenue that links Bogota's center with its international airport.

Moreno and Petro had both been members of the leftist Polo Democratico Alternativo party, of which Petro was a founder in 2005.

Moreno was expelled from the party and Petro distanced himself from it, creating a "Progressives" movement.

Ironically, Petro's mother was a champion of Moreno's maternal grandfather, Gen. Gustavo Rojas Pinilla, the populist candidate on the losing end of the highly questioned 1970 election.